Ransomware is a denial of access attack on a computer system. In a typical ransomware attack, a malicious Trojan is installed on the target machine. The Trojan then locks down the computer, and demands a ransom for unlocking it. An unsophisticated ransomware may use an easily-circumvented attack, such as locking the user's account. But more advanced ransomware may actually encrypt the user's files, and demand a ransom in exchange for the decryption key. As it may be impractical to decrypt the files without the key, and as the user's files may be important, many users pay the ransom.